I’m surprised that you didn’t get a check engine light with the oil cap off. Years ago, I left the oil cap off on my 2000 F150 after changing the oil and I had the CEL a day or two later. The code was something about a massive vacuum leak. In your case, I agree with the previous poster that Midas probably only installed 5 quarts of oil. I'm glad you got the oil and filter change done, you should be good now.